William Hague has pleaded for Sunday's referendum on Crimea's future to be called off and warned Russia that European travel bans and asset freezes will be imposed unless progress is made on tackling the Ukraine crisis.
Ahead of talks in London between US secretary of state John Kerry and Russian counterpart Sergey Lavrov, Mr Hague said the referendum would not be "legal or legitimate".
The Foreign Secretary said: "Part of the answer to finding ways of de-escalating is for this referendum to be called off and for international negotiations to take place.
"If there is no progress on these things in the next few days then we will move to the next stage of Europe's response to this crisis which will involve travel bans and asset freezes on individuals in Russia.
"That will be discussed by European Union foreign ministers in Brussels, I will join that meeting, on Monday."
Prime Minister David Cameron and Mr Hague will meet Mr Kerry before his talks in London.
Mr Hague said Europe had made clear that a "further intensification" of the crisis could lead to "more far-reaching" action by the EU than just travel bans and asset freezes.
"But the emphasis in this coming 24 hours and with these talks in London on Friday is to do everything possible to find a diplomatic way forward and to try to make sure that every possible avenue for de-escalating this tense situation is found," he said.
If the referendum went ahead it "almost certainly does trigger stage two" of the EU's three-phase sanctions plan, which could see Russian politicians and officials targeted with asset freezes and travel bans.
Any escalation in diplomatic and economic pressure against Russia could lead to retaliation by Moscow, with European gas supplies and investment in financial centres such as the City of London potentially vulnerable.
Mr Hague said: "European countries do understand that there can be consequences in both directions of measures that we take.
"But it is very important to be clear about what we think of the violation of the sovereignty, independence and territorial integrity of another nation.
"It's important to take a united stand together but at the same time, and I stress this, it's very important to use every possible diplomatic avenue to seek an improvement in the situation and agree the way forward, to keep in full communication with Russia about the situation."
He said Mr Cameron and Russian president Vladimir Putin had spoken to each other four times since the crisis began. But in a sign of frustration with Moscow's attitude, asked whether he sensed any softening of the mood in the Kremlin he said: "Russia hasn't made any concessions on any of the things of concern to Ukraine or to Western nations.
"They haven't made any concessions on that so far.
"They have not used any opportunity to de-escalate the situation themselves. So these talks tomorrow will be a crucial moment for discovering, given the effective deadline of Sunday's referendum, whether they really do want to de-escalate the situation."